1. Field of the Invention
The present invention relates to a solid electrolyte, especially to a stabilized zirconia solid electrolyte, having an improved mechanical strength without a lowering of the oxygen ion conductivity, and a process for the preparation thereof.
2. Description of the Related Art
Stabilized zirconia is used as the electrolyte of a solid oxide fuel cell. In stabilized zirconia, a tetragonal zirconia phase (partially stabilized zirconia) is formed in the region where the amount of a stabilizer is small, and as the amount of the doped stabilizer is increased, a cubic zirconia phase (fully stabilized zirconia, hereinafter referred to as "stabilized zirconia") is formed. An element such as yttrium, cerium, calcium or magnesium is used as the stabilizer.
Both stabilized zirconia and partially stabilized zirconia have an oxygen ion conductivity, but stabilized zirconia has a higher ion conductivity, and stabilized zirconia has a superior stability at high temperatures. On the other hand, partially stabilized zirconia is a ceramics material having a very high strength and partially stabilized zirconia is much better than stabilized zirconia as a construction material. Accordingly, in industry, stabilized zirconia is used for an element having a small volume, for example, an oxygen sensor, in view of its electroconductivity, and partially stabilized zirconia is utilized for a pulverizing ball or a zirconia kitchen knife, in view of its strength.
In the case of a solid oxide fuel cell, to reduce the internal electric resistance of the cell, an electrolyte having a high oxygen ion conductivity is preferred, and the stabilized zirconia is generally used.
As the structure of the solid oxide fuel cell, there have been proposed a tubular structure and a planar structure. The tubular structure is constructed by the method in which a cell (electrolyte and electrodes) is supported on a porous ceramics supporting tube, and in this case, since self-supporting of the cell is not necessary, the required strength condition is not so strict. Nevertheless, since the volume of the support makes no contribution to the generation of electricity, a high integration is difficult.
In the case of the planar structure, since no support is disposed for the cell, a high integration is possible, but for the self-support of the cell, the strength condition required for the cell, especially the electrolyte, is very severe.
Accordingly, for the electrolyte of the planar solid oxide fuel cell, a utilization of partially stabilized zirconia or an increase of the strength by adding 5 to 30% by weight of an alumina powder to a stabilized zirconia powder as the starting material has been proposed.
When partially stabilized zirconia is used as the electrode, a problem arises in that the internal electric resistance of the cell is increased, compared with the internal electric resistance observed when stabilized zirconia is used. According to the method in which an alumina powder is added to a stabilized zirconia powder as the starting material, the strength of the solid electrolyte is improved, but since alumina is an insulating substance, a problem of a drastic reduction of the oxygen ion conductivity arises.
With this background, a primary object of the present invention is to provide a solid electrolyte in which the strength is improved without a lowering of the high ion conductivity of stabilized zirconia, and a process for the preparation thereof.